1. Field of the Invention
The present invention relates to a calibration method for correcting the image density and color tone, an image forming system, an image forming apparatus, and a calibration program for calibration operation.
2. Description of Related Art
A great number of image forming apparatuses based on electrophotographic technology such as copiers, printers and facsimiles have been introduced in offices. The image forming apparatus is required to reproduce a high-quality image having uniform glossiness. The image formed by electrophotographic technology is formed by toner particles, and roughened structures are produced on the image surface, with the result that uniform glossiness is not likely to occur. To improve this point, it will be possible to work out a heat fixing step and fixing a toner image on a sheet of paper. The image formed by electrophotographic technology, however, contains a toner-rich area and a toner-poor area. Working out of a fixing step cannot succeed in complete elimination of the roughened structure of the image face. Further, the toner-rich area rises to form a relief. Such a problem cannot be solved by only working out a fixing step.
In an effort to solve this problem, Unexamined Japanese Patent Application Publication No. 7-56409 discloses a technique wherein a mirror image is transferred and fixed onto a transparent sheet, and a light reflecting member is bonded to the toner image carrying face of the transparent sheet. According to this technique, the toner image is sandwiched between the transparent sheet and light reflecting member. The face opposite to the toner image carrying face of the transparent sheet becomes the image front. To be more specific, the image is viewed through the transparent sheet. Since the transparent sheet has a smooth face, an image having uniform glossiness can be provided by electrophotographic technology as a matter of form.
Incidentally, in the image forming apparatus based on electrophotographic technology, especially in the color image forming apparatus, in order to ensure adequate density, color tone and lightness of the output image, calibration is performed in such a way that the measured data (e.g. calorimetric data) is converged on the target data (e.g. target color data) so that the output image is read by a reading section such as a scanner densitometer and calorimeter and an ideal output value is obtained. According to the conventional calibration, an image is formed on white paper. The face where this test image is formed is used as a face to be read, and the test image is read by the reading section. However, as in the aforementioned Patent Document 1, when a mirror image is formed on the transparent sheet and is observed from the back, even if the face of the transparent sheet with the image formed thereon is read by the reading section such as a scanner according to the conventional calibration method, the measurement data with consideration given to the properties of the transparent sheet cannot be obtained. Thus, density or color tone cannot be adjusted properly.
In an image forming apparatus capable of forming a normal image on the standard paper or an mirror image on the transparent sheet by user's mode selection, when calibration of density and color tone is carried out, it is necessary to distinguish between the calibration to be performed for forming a normal image on the standard paper, or the calibration to be performed for forming an mirror image on the transparent sheet. Otherwise, a wrong correction data may be used for image formation, and an image characterized by proper density and color tone cannot be obtained.
Thus, one of the objects of the present invention is to provide calibration operation with consideration given to the properties of the transparent sheet. Another object of the present invention is to provide proper calibration operation in an image forming apparatus capable of forming a normal image on the standard paper and a mirror image on the transparent sheet, wherein a clear distinction can be made between calibration for the normal image or calibration for the mirror image.